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Cisco CCNA 640-802 / CCENT Certification Exam Training :
Spotting And Troubleshooting OSPF Network Type Mismatches
New CCNA Training Videos And Video Practice Exams Have Been Added To The End Of This Tutorial!
By Chris Bryant, CCIE #12933
When you configure OSPF on a point-to-point Frame Relay subinterface, the OSPF network type will default to point-to-point. That certainly makes sense!
However, there can be a problem with such an interface when it attempts to form an adjacency with an OSPF-enabled interface that's running a different network type. In the following exhibit, R1 is running OSPF on its physical Serial0 interface, and R3 is running OSPF on its S0.31 point-to-point interface.
We already have a successful adjacency over this hub-and-spoke network between R1 and R2, so we must keep that in mind when discussing a potential solution to the current adjacency issue.
I'll take this opportunity to introduce you to the show ip ospf interface command.
R1#show ip ospf interface serial 0
Serial0 is up, line protocol is up
Internet Address 172.12.123.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type NON_BROADCAST, Cost: 64
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 172.12.123.1
No backup designated router on this network
Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
Hello due in 00:00:23
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 2, maximum is 2
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 2.2.2.2
Suppress hello for 0 neighbor(s)
R3#show ip ospf interface serial 0.31
Serial0.31 is up, line protocol is up
Internet Address 172.12.123.3/24, Area 0
Process ID 1, Router ID 3.3.3.3, Network Type POINT_TO_POINT, Cost: 64
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:04
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 0, maximum is 0
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 0, Adjacent neighbor count is 0
Suppress hello for 0 neighbor(s)
See the problem? The Hello and Dead timers don't match. When you configure OSPF on a point-to-point link, the interface naturally defaults to an OSPF point-to-point network... and the hello and dead timers on that network type are 10 and 40, respectively.
We've got to change that before an adjacency can possibly form. We have two options:
Use the ip ospf hello command to change the hello timer on R3 (If we change it on R1, we'll lose the adjacency we already have to R2)
Use the ip ospf network command to change R3's OSPF network type on that subinterface to non-broadcast, which will make it match R1's hello and dead timers
In this case, we'll use the ip ospf network command.
We'll verify the changes with show ip ospf interface....
R3#show ip ospf interface serial0.31
Serial0.31 is up, line protocol is up
Internet Address 172.12.123.3/24, Area 0
Process ID 1, Router ID 3.3.3.3, Network Type NON_BROADCAST, Cost: 64
Transmit Delay is 1 sec, State DROTHER, Priority 0
No designated router on this network
No backup designated router on this network
Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
Hello due in 00:00:25
.... and shortly after that change, the OSPF adjacency between R1 and R3 comes up. R3 sees R1 as the DR on this hub-and-spoke network, and R1 sees R2 and R3 as neither a DR nor a BDR - in other words, they're "DROTHERS".
R3#show ip ospf nei
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/DR 00:01:36 172.12.123.1 Serial0.31
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/DROTHER 00:01:51 172.12.123.2 Serial0
3.3.3.3 0 FULL/DROTHER 00:01:38 172.12.123.3 Serial0
There's nothing like seeing all of this theory in action on live Cisco routers -- and right after this brief and important message, there are four videos that will do just that for you!
